riscv: align struct __esf properly

The minimum stack alignment is 16. Therefore, the stack space to store
a struct __esf object must be rounded up to the next 16-byte boundary.

It is not sufficient to do the rounding on the __z_arch_esf_t_SIZEOF
definition. When the stack is constructed in arch_new_thread() it is
also necessary to do the rounding there too.

Let's make the structure itself carry the alignment attribute instead to
make it work in all cases.

While at it, remove the unused _K_THREAD_NO_FLOAT_SIZEOF definition.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>